Final answer:
An underwater avalanche is scientifically known as a turbidity current, which leads to the formation of turbidite sedimentary deposits on the sea-bottom.
Step-by-step explanation:
An underwater avalanche is called a turbidity current. This geological phenomenon occurs when a mass of sediment laden water, moved by gravity, flows downslope on the seabed. Turbidite sedimentary deposits are formed by these underwater avalanches due to massive slope failures. The currents are capable of carrying grains sorted by density, forming distinctive layers on the sea-bottom. This is different from a transgression, which refers to areas of land being submerged below sea level due to sea level rise, and is not directly associated with underwater avalanches.